What BMI tells you — and what it doesn’t

Body mass index is weight in kilograms divided by height in meters squared. It is the most widely used number for classifying weight — and one of the most criticised. Both views are right: BMI is useful for what it was designed to do, and misleading when it is asked to do more.

Where BMI came from

The ratio was described in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the “average man”, not individual health. In 1972 the physiologist Ancel Keys and colleagues compared several weight-for-height indices in 7,424 men across five countries and found that weight ÷ height² tracked body fat better than the alternatives while being largely independent of height. They named it the body mass index.

What it is good for

• Screening large groups. Across populations, higher BMI is associated with higher rates of type 2 diabetes, high blood pressure and heart disease. Public health agencies use it to track trends.
• A quick first check. It needs only a scale and a tape measure, costs nothing and is easy to repeat.
• Tracking change in one person. If your muscle mass is stable, a rising BMI over time usually means rising body fat.

What it misses

BMI cannot tell fat from muscle, bone or water, and it says nothing about where fat is stored. That leads to predictable errors:

• Muscular people can be classed as overweight with low body fat.
• Older adults who have lost muscle can have a “healthy” BMI with a high fat percentage.
• Health risks begin at lower BMI in some populations. The UK’s NICE guideline uses lower thresholds — overweight from 23 and obesity from 27.5 — for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
• For children and teenagers, BMI must be compared with growth charts by age and sex.

Waist-to-height ratio: a simple addition

Fat stored around the abdomen carries more risk than fat elsewhere. NICE recommends measuring waist-to-height ratio alongside BMI for adults with a BMI under 35, with a memorable target: keep your waist to less than half your height.

Someone 69 inches tall with a 34-inch waist has a ratio of 34 ÷ 69 = 0.49, just under the 0.5 boundary. NICE classes 0.5 to 0.59 as increased central adiposity and 0.6 or more as high. Unlike BMI, the same boundary is used for all sexes and ethnicities.

Measure your waist halfway between the bottom of your ribs and the top of your hips, after breathing out.

Using the numbers together

No single measure describes health. BMI, waist-to-height ratio, blood pressure, blood sugar, cholesterol, fitness and family history each add a piece. A result outside the healthy range is a prompt for a conversation with a doctor, not a verdict.